As every gardener knows, when plants are too close together the crowding elicits a number of developmental responses, such as stem and petiole elongation, branch suppression and accelerated flowering (Smith, H. 1982, Light quality, photoreception and plant strategy. Annu. Rev. Pl. Physiol. 33: 481–518 and Schmitt, J. and R., D., Wulff 1993, Light spectral quality, phytochrome and plant competition. Trends Ecol. Evol. 8:47–50). This shade avoidance response is triggered by the reduced ratio of red to far red wavelengths (R:FR) transmitted through or reflected from green vegetation due to selective absorption of visible wavelengths by chlorophyll (see Smith 1982, above).
It is the phytochrome family of photoreceptors that senses these environmental variations in the R:FR ratio. Phytochromes reversibly switch between R and FR-absorbing forms and interacts with multiple signaling pathways, such as the auxin pathway, to provide a dynamic response to shade (see Smith 1982 and Smith, H. 1995, Physiological and ecological function within the phytochrome family. Annu. Rev. Plant Physiol. Plant Mol. Biol. 46:289–315).
Shade tolerance, or the ability to tolerate extended periods of low light, varies from species to species. Photosynthesis is decreased in shade. As a consequence, in species such as turf grasses this results in decreased carbohydrate reserves and reduced root, rhizome and tiller growth. In the turf grass industry this is problematic because about 20–25% of turf grasses are grown under low light conditions and a considerable amount of time and money is spent by golf courses in an effort to maintain quality turf under shade conditions.
Shade intolerance (shade avoidance) is detrimental to crop plants because the growth and performance of crop plants depends largely on crop architecture, and plant architecture is affected by reduced light. That is, densely planted crops that shade one another tend to place energy into stem and petiole elongation to lift the leaves into the sunlight rather than putting energy into storage or reproductive structures. This negatively affects yields by reducing the amount of harvestable products such as seeds, fruits and tubers. In addition, tall spindly plants tend to be less wind resistant and fall over easily, further reducing crop yield.
Likewise, shade intolerance negatively affects forestry plantings. Here, seedlings of shade tolerate species will self-prune at a slower rate and survive for longer periods under a dense forest canopy than shade intolerant trees. Since most commercially important tree species are shade intolerant to only moderately tolerant of shade, tree plantings must be less dense and require increased acreage.
In the field of agriculture efforts are constantly being made to produce plants with an increased growth potential in order to feed the ever-increasing world population. A similar effort is underway in the field of forestry with the goal of guaranteeing a supply of reproducible raw materials. Conventionally, plant improvement has been achieved via plant breeding. The breeding process is, however, both time-consuming and labor-intensive, especially in forestry where trees may not reach reproductive maturity for decades. Furthermore, appropriate breeding programs must be performed for each relevant plant species.
The genetic manipulation of plants has expedited progress by introducing and expressing specific recombinant nucleic acid molecules. This approach has the advantage of being generally transferable among plant species rather than being limited to one plant species. For example, EP-A 0 511 979 describes the expression of a prokaryotic asparagine synthetase gene in plant cells that leads to increased biomass production. Likewise, WO 96/21737 describes plants with increased yield (growth potential) arising from an increase in the photosynthesis rate and the expression of deregulated or unregulated fructose-1,6-bisphosphatase. Nevertheless, there still is a need for generally applicable processes that improve agricultural and forest plant growth potential. Therefore, the present invention relates to a process for increasing the growth potential in plants, characterized by expression of recombinant DNA molecules stably integrated into the plant genome, particularly in combination with the shade responsive promoters of the invention.